Pulse width modulation (PWM) is a conventional technology used for controlling power converters to achieve output power, voltage, and current regulation. Conventional flyback power converters include a power stage for delivering electrical power from a power source to a load, a switch in the power stage that electrically couples or decouples the load to the power source, and a switch controller coupled to the switch for controlling the on-time and off time for the switch. The on-time (TON) and off-time (TOFF) for the switch can be modified by the controller based on a feedback signal representing output power, voltage, or current. The energy is stored in the transformer core gap when the switch is on, and is transferred to the load circuit when the switch is off. Regulation can be accomplished by, among other things, measuring the output power, voltage, or current, and feeding an indicating signal back to the primary side controller, which can modify the TON-time and TOFF-time of the switch accordingly to effectively regulate the output power, voltage, or current. The switching cycle TS is the sum of the on-time TON and off-time TOFF.
In power supply designs, it is necessary to regulate power, voltage, or current depending upon the application. It is also desirable to regulate the output voltage at the end of a cable attached to the power supply, rather than to merely regulate the voltage at the output of the supply. The present invention is related to the regulation of voltage and current at the end of the cable attached to the supply where the resistance of that cable is a known value when the controller is a primary side controller.
One conventional power supply system involves a flyback converter that senses the output voltage directly from the secondary side of the transformer. This is called secondary sensing. FIG. 1 is an illustration of such a conventional secondary side sensing circuit.
FIG. 1 illustrates a traditional flyback power supply with secondary sensing. It is configured to regulate both voltage in constant voltage (CV) mode and current in constant current (CC) mode. The PWM controller 100 is powered by Vcc which can be derived a number of different ways from the power supply. When the PWM controller begins operation, it outputs a PWM stream to MOSFET 120, which turns on the primary current of transformer 110. As the flyback operation takes place, energy is transferred from the primary side 111 to the secondary side 112 of the transformer 110 during each cycle that over time constitutes an output power to be dissipated over the Load 160. As the output voltage exceeds the sum of the zener diode 140 voltage and the drop across the forward biased diode 152, which is part of the opto-coupler 150, the opto-coupler diode 152 conducts, and turns on the NPN photo-transistor 154 that is part of the same opto-coupler 150 integrated circuit. When the transistor 154 is turned on, this draws current that causes the controller to reduce the TON-time. In addition, there is a current sense resistor 170 that will develop a voltage drop across the base-emitter junction of transistor 130. When the load resistance 160 is decreased such that the power supply delivers the maximum current, the transistor 130 bypasses the zener diode 140, and causes current regulation.
Note in FIG. 1 that the voltage sense resistor 170 is connected to the output of the secondary of the supply. If there is a resistive cable between the output and the resistive load, the point of regulation can be moved to the load side of the cable to accomplish regulation at the destination side of the cable. This is shown in FIG. 2. The resistive load 160 is now separated from the supply by the equivalent series resistances of each half of the delivery cable 280 and 281. To cause the regulation to compensate for the resistance of the cable, the points of regulation are moved to the destination end of the cable. This has the very costly problem of having to have 4 wires in the delivery cable, which is a significant cost issue in small applications. This method of regulation is typically known as remote sensing.
There are at least two significant drawbacks in cost to this secondary side regulation solution, even when there is no requirement for “remote sensing”, or cable end regulation. First, it requires the external circuit consisting of the transistor 130 and the current sense resistor 170 to aid the current regulation. The second drawback is the wasted power dissipated by the sense resistor, which reduces the overall system efficiency.
FIG. 3 is an illustration of a conventional primary side sensing regulation circuit that overcomes some of the problems described above. It accomplishes the same regulation functions, but the feedback for regulation is taken from the transformer on the primary side of its safety isolation barrier.
In FIG. 3, there is a fly-back transformer 370 with primary winding 371 and secondary winding 372. For primary feedback control, there is an additional winding 373 added to the transformer installed on the same side of the safety isolation barrier as the primary winding. When the controller 300 outputs a pulse to the switching MOSFET 355, the current in the primary 371 of the transformer ramps up linearly for the duration of the pulse. When the switch 355 is turned off, the collapsing field is coupled to the output or secondary winding to transfer energy to the secondary.
In a flyback type power converter that operates in discontinuous conduction mode, the output power P0 can be expressed as:
                              P          o                =                                            Vin              2                                      2              ⁢                              L                M                                              ×                                    t              on              2                                      T              S                                ×          η                                    (        1        )            
Where η is the power efficiency (Po/Pin), and
The output voltage is thus expressed as:
                    Vo        =                              Vin            ·            Ton                    ⁢                                                    R                                  2                  ⁢                                      L                    M                                    ⁢                                      T                    S                                                                        ·            η                                              (        2        )            
The operation of the system shown in FIG. 3 is now set forth. The auxiliary winding of the transformer 373 feeds a voltage reflection of the output secondary that is used to track the output voltage. This can be done with various sensors 320 that know how to analyze the voltage waveform from the auxiliary winding 373. One example of a such a sensor 320 (also known as a GAP detector) is described in U.S. Pat. No. 6,956,750 which is incorporated by reference herein in its entirety. The output of that sensor 320 is subtracted by the subtraction unit 325 from a digital reference level (Vref) that corresponds with the ideal regulated output voltage. The output of the subtraction unit 325 is called an error value, and it drives a function called 310 the Digital Error Amplifier (DEA). This is essentially the system loop filter, which can contain various combinations of digital filters to process the error and create a digital Control Voltage (called Vc). In one embodiment of the present invention, the DEA 310 is a Proportional-Integral (PI) filter. This output goes to a time calculator (340) which calculates the on time for the switch 355 in the fly-back power converter. The output of this calculation drives a timer (350) which in turn creates the PWM pulse that drives the switch 355.
The Digital Error Amp 310 contains an integrator (Proportional-Integrator embodiment) such that the output will become steady state when the input error from the subtraction unit 325 is equal to 0. Therefore one can see that the Ton time is varied through the loop control to maintain a voltage that is represented by the Vref input to the error subtraction 325.
What is needed is a modification to the primary side sensing control system that (1) provides a method for compensating the output voltage for the amount of voltage dropped in the delivery cable, and (2) does so without requiring any other sensor feedback other than the already existing auxiliary winding input from the transformer.